The present invention is directed towards an implantable stimulation device, such as a cardiac pacemaker or an implantable cardioverter-defibrillator device, which has a system that automatically and reliably adjusts its sensitivity for sensing cardiac signals and for reliably discriminating between P-waves, R-waves and T-waves, even in the presence of noise and complete heart block.
Every modern implantable stimulation device includes sensing capability, whether in one or two chambers of the heart. Typically, sensing of the low amplitude cardiac signals is achieved by a pre-amplifier and a comparator which detects if a predetermined threshold have been exceeded. The xe2x80x9csensitivityxe2x80x9d is generally thought of as a measure of the level, in millivolts, in which a cardiac signal must exceed in order to be detected by the sensing circuitry. If the sensitivity is too low (i.e., too insensitive), then some cardiac events will not be detected. If the sensitivity is too high, then erroneous sensing of noise or undesired cardiac signals may result (e.g., the double sensing of T-waves may occur, noise may be mistaken for P-waves, and far-field R-waves may be mistaken as P-waves, etc.).
Systems for automatic sensitivity control in an implantable stimulation device have long been plagued by the inability to reliably detect the low amplitude P-waves signals (typically about 4 mV) in the presence of noise and myopotentials, and by the inability to discriminate P-waves and R-waves from T-waves. Although a physician may be able to readjust the sensitivity of the device, many patients are unable to see their physicians frequently enough. This problem is compounded when several other factors may also affect the device""s sensitivity as a function of time, such as fibrotic tissue growth, drugs, dislodgment, arrhythmias, changes due to defibrillation shock, etc. Thus, it is desirable to have a system that can automatically and reliably adapt itself as the patient""s needs change.
Many schemes have been attempted which require additional sensing amplifiers and hardware, which in turn require more real estate on the IC""s and more current drain. For example, Hamilton et al. (U.S. Pat. No. 4,708,144) discloses a system for measuring a peak R-wave and deriving a long term average to determine the gain setting and threshold of the amplifier. This configuration requires a conventional amplifier and a comparator, in addition to, an attenuator, a filter, an A/D converter, an absolute value circuit and a peak detector.
Decote, Jr. (U.S. Pat. No. 4,768,411) discloses a dual comparator system, having two different thresholds but coupled to the same input, which simultaneously adjusts each threshold until one comparator senses the cardiac signal and one does not. In addition to a conventional amplifier and comparator, this system requires a precision signal rectifier, a D/A converter, a voltage divider, and a second comparator. The reference by Schroeppel (U.S. Pat. No. 4,766,902) teaches a similar approach albeit with less specificity.
Baker, Jr. et al. (U.S. Pat. No. 4,903,699) discloses an even more complicated scheme using a quad-comparator system for separately determining sensing threshold and amplifier gain. This system requires an automatic gain control circuit, three amplifiers, and four comparators.
It is also known that low amplitude ventricular fibrillation (VF) can go undetected if the sensitivity level (in mV) is set for normal bradycardia activity. If the sensitivity level is set too high (i.e., too sensitive), however, then double sensing of T-waves can result in erroneously classifying a rhythm as VF. Also, it is known that, post defibrillation shock, the current of injury T-wave can be higher in amplitude than R-waves. The Grevis et al. reference (U.S. Pat. No. 4,940,054) discloses a system for setting the sensing circuitry to any one of a multiple sensitivities depending on the type of rhythm detected, or expected, based on the present therapy.
Different ones of the electrode configurations have proven more useful than others for sensing certain cardiac events. Briefly, the benefits of bipolar leads are well known: for example, capture detection is superior when used with bipolar electrodes. Also, it is well known that bipolar stimulation can eliminate pectoral stimulation; bipolar sensing can eliminate sensing of noise and EMI. In some instances, unipolar sensing may provide a larger cardiac signal based on the vector; and unipolar stimulation may avoid diaphragm stimulation in some patients. Implanting bipolar leads enables complete programmable polarity. Thus, if the patient needs a different electrode configuration to avoid a pacing or sensing problem, or if a wire fractures, the physician can reprogram the electrode configuration to an alternative pair of operable electrodes. The need to change sensing electrode configurations provides yet another reason for automatically determining a new sensitivity setting, and this need is magnified if the implanted device can perform this reprogramming automatically based on predefined criteria (e.g., an improper lead impedance, a change in the rhythm, etc).
The problem of accurately automatically sensing P-waves and R-waves is even more pronounced when using an xe2x80x9cA-V combipolarxe2x80x9d electrode configuration, that is, an electrode configuration in which the stimulation device senses cardiac signals between an atrial tip electrode and a ventricular tip electrode, and stimulates each chamber in a unipolar fashion from the respective electrode to the housing (i.e., typically referred to as the case electrode). For a more complete description of combipolar systems, see U.S. Pat. No. 5,522,855 (Hognelid), which reference is incorporated herein by reference. When such electrodes are implanted, various electrode sensing configurations are possible, e.g., atrial unipolar (Atip-case); ventricular unipolar (Vtip-case); atrial-ventricular combipolar (Atip-Vtip); ventricular unipolar ring (Vring-to-case) or atrial unipolar ring (Aring-to-case).
Regardless of the cardiac event being sensed, however, and regardless of the electrode configuration being used, there is a need for the implantable device to be able to readily and reliably distinguish between P-waves, R-waves and T-waves. This is because the implantable device, if it is to perform its intended function, must know when an atrial depolarization occurs (P-wave), and when a ventricular depolarization occurs (R-wave), and it must not be fooled by falsely sensing a T-wave or noise as a P-wave or R-wave.
For example, it is of critical importance that the device be capable of recognizing the occurrence of certain atrial arrhythmias based on the sensed atrial rate, and in determining such rate it is critically important that neither R-waves nor T-waves be falsely sensed as a P-wave. Such may be particularly noticeable when an A-V combipolar electrode configuration is being used because, in such configuration, P-waves, R-waves, and T-waves may be sensed as being of the same order of magnitude. This problem may be made even more difficult during a mode switch, e.g., when switching a pacemaker from a DDD mode to a VVI or DDI mode, because such a mode switch tends to introduce retrograde P-waves, of which occurrences may be sensed and falsely assumed to be an antegrade P-wave.
While it is well known that various blanking schemes may be used to block or blank out unwanted T-waves and retrograde P-waves by using different blanking intervals (i.e., PVARP, automatic PVARP extension, PVAB, etc.), and thereby prevent such T-waves or retrograde P-waves from being falsely sensed as P-waves, such blanking schemes (based solely on timing considerations) have proven less than satisfactory because legitimate (antegrade) P-waves (which need to be sensed) may and do occur during these blanking intervals.
Differentiation schemes based on the morphology of the sensed waveform have also been used. Such schemes are premised on the fact that P-waves, R-waves and T-waves have inherently different shapes. Thus, in theory, all one needs to do is to examine the morphology of the sensed waveform. Unfortunately, morphology-based schemes require that the entire waveform be captured and analyzedxe2x80x94a process that not only requires waiting until the entire waveform has occurred, but also may require significant on-chip processing capability and processing time.
Thus, it is seen that there is a need in the implantable cardiac stimulator art to accurately detect and discriminate between P-waves, R-waves and T-waves, without relying solely on blanking considerations or morphology. This need is particularly acute when sensing between intra-chamber electrodes, e.g., when sensing using an A-V combipolar electrode configuration.
The disadvantages and limitations of the background art discussed above are overcome by the present invention. The present invention contemplates a system and method for automatically determining the sensitivity setting for a sense amplifier by determining the threshold for sensing of undesired signals (e.g., T-waves, far-field R-waves, etc.).
Broadly, the sensing threshold for the undesired signal may be achieved either by directly measuring the peak amplitude of the undesired signal (e.g., using an A/D converter or a peak detect circuit), or by indirectly measuring the peak by detecting the presence and then the absence of the undesired signal. In order to detect the undesired signal, the present invention analyzes the time-relationship between the two or more cardiac signals.
More specifically, in one embodiment used within the ventricular channel, the sensing level for detecting an R-wave and for discriminating against T-waves may be determined by setting an adjustable sensitivity (i.e., as determined by the threshold and gain settings) of a programmable gain sense amplifier to its highest setting (e.g., its highest gain and lowest threshold settings) so that double sensing of the R-wave and the T-wave occurs. Then, the sensitivity may be incrementally decreased until only single sensing of the R-wave occurs.
An adequate safety margin is preferably added to the sensitivity setting to avoid beat-to-beat variability of the T-wave, and such sensitivity setting is thereafter used to ensure that only single sensing occurs, i.e., to ensure that only the R-wave and not the T-wave is sensed.
In a variation of the above embodiment, the system could continue adjusting the sensitivity setting to determine the peak R-wave. The system preferable includes also adding a safety margin below the peak R-wave to avoid beat-to-beat variability of the R-wave. The system may also store the amplitude and the time-relationship between the peak R-wave and the peak T-wave for statistical analysis.
In another embodiment used within the atrial channel, the sensing level for detecting P-waves and for discriminating against far-field R-waves and T-waves may be determined by setting an adjustable sensitivity setting of a programmable gain sense amplifier to its highest sensitivity setting so that triple sensing occurs, i.e., P-waves, R-waves and the T-waves are sensed simultaneously. Then, the sensitivity setting may be incrementally decreased until only single sensing of the P-waves occur. The processor then characterizes and stores the time-relationship and amplitude of each signal as each peak is detected so that proper classification of the cardiac signals can occur.
An adequate safety margin is then added to the atrial sensitivity setting, similar to that described above for R-wave sensing, and such atrial sensitivity setting is thereafter used to ensure that only single sensing occurs.
While it may appear that above embodiment presumes that the far-field R-wave and T-wave are smaller than the P-waves, even if this were not true, the processor can easily determine which signals correspond to P-waves by examining the time-relationship of each peak.
Such is the case when the present invention is used with an A-V combipolar electrode configuration, wherein R-waves are generally larger than P-waves and T-waves, but T-waves may at times be larger, the same, or smaller than P-waves. The processor simply characterizes and stores the time-relationship and amplitude of each signal as each peak is detected. After analyzing the time-relationship, the system can identify which peak corresponds to the P-wave and then add an adequate safety margin to it.
Thus, even if the patient is in complete heart block (where there is total disassociation between P-waves and R-waves), the system can reliably detect P-waves since the time-relationship between R-waves and T-waves is, physiologically, fixed. That is, the P-waves may have a variable time-relationship with respect to the R-wave, but the T-wave will always be coupled to the R-wave and fall within an expected interval. Likewise, in the instance where current of injury T-waves are larger than R-waves, the system can reliably detect R-waves based on their time-relationship.
In any of the above-described embodiments, an A/D converter or peak detector may be used to directly measure the peak cardiac signal and classify the cardiac signals as P-waves, R-waves, and T-waves based on their time-relationship and amplitude.
In yet another embodiment, the present invention differentiates P-waves, R-waves and T-waves by analyzing amplitude, duration and time-relationship for each. That is, the analysis includes measuring the amplitude and duration of a sensed signal, determining if the amplitude and duration qualify these signals as being legitimate, and then ascertaining whether the measured signal is a P-wave, R-wave or T-wave as a function of the time-relationship and amplitude within the cardiac cycle during which the peak amplitude occurs.
In yet another embodiment, the system may store a histogram of the frequency of cardiac signals having a particular time-relationship and amplitude. Thus, not only are intrinsic signals (R-waves, P-waves, and T-waves) mapped into the histogram, but paced signals and far-field signals can be separately characterized and have, effectively, their own signature. Premature beats can either be ignored or likewise characterized.
The present invention further contemplates a robust method for statistically analyzing the time-relationship and classification of P-waves, R-waves and T-waves and thereafter adjusting the sensitivity based on such classification and amplitudes.